Calcium carbonate can be precipitated in three different crystalline forms--calcite, aragonite or vaterite, although calcite is the only stable form of calcium carbonate at room temperature and atmospheric pressure. Provided the three crystalline forms are of adequate purity, all three crystalline forms or mixtures thereof can be satisfactorily used to produce high quality fluorescent lamp phosphors. An important consideration in this respect is the sodium content of the calcium carbonate, because sodium quenches the brightness of fluorescent phosphors.
Because vaterite is a scavenger of sodium and other cations, it is not normally used as a raw material for halophosphate type phosphors. U.S. Pat. No. 3,578,603 dated May 11, 1971 issued to E. A. Dale et al., however, teaches the preparation of a low sodium type vaterite and claims brightness gains in its corresponding phosphors as a result of the high reactivity of its meta-stable crystalline structure. It further teaches that to insure the formation of high purity vaterite, raw materials for its manufacture must be prepared in a certain manner so that sodium does not enter into them. Clearly, the use of an inexpensive raw material, such as calcium chloride which is a by-product of the Solvay process for producing soda ash which has a typical analysis of 1.6% sodium chloride, is precluded.
Aragonite is also thermodynamically unstable; however, it does not scavenge sodium ions from solution. Aragonite though is economically unattractive in that it requires high precipitating temperatures as well as high pH values to precipitate pure aragonite.
For the foregoing reasons and because it forms easily and has no particular affinity for sodium, calcite is the usual form in which phosphor grade calcium carbonate is prepared. Disclosed in the present application is an inexpensive and commercially practical method of making a very low sodium, finely-divided phosphor grade calcium carbonate.